Stuck in the desert with a drug lord in their car boot, Barnaby and Nina are in a real mess. The pair are even more worried when they learn that the Sherman twins have put a price on their heads.
Nina and Barnaby attempt to escape from Mexico. With no money and both the drug cartel and the Sherman Brothers' own bounty hunter on their tail, they need all their cunning and wit to make it safely back to England.
Nina wants the reward for finding the Hitler painting, so she drugs Barnaby and takes him to the Sherman Twins. Barnaby agrees to help find the picture, so he can part company with Nina forever.
Barnaby has been taken prisoner by Colin McQueen, who is also looking for the Hitler painting. Nina and Leah work together to find the picture and free Barnaby, while keeping the Sherman Twins' bounty hunter at bay.